The present invention relates to a semiconductor laser and particularly to a semiconductor laser whose active layer is made of a II-VI group compound semiconductor material and a manufacturing method thereof.
It is difficult to obtain a satisfactory ohmic contact at a p side in a semiconductor laser in which a II-VI group compound semiconductor material of ZnSe system or the like is used. Therefore, there are the disadvantages in the prior art that generation of heat at a contact portion causes a deterioration of an electrode, that a rise in temperature of an element makes long-time operation difficult, or the like.
Construction of the ZnSe system semiconductor laser as an example will be explained with reference to FIGS. 1A to 1C showing manufacturing processes thereof. In this case, first, as shown in FIG. 1A, a first cladding layer 2 made of an n type ZnMgSSe or the like, an active layer 3 made of an undoped ZnCdSe or the like, a second cladding layer 4 made of a p type ZnMgSSe or the like, a buffer layer 5 made of a p type ZnSe or the like and further a contact layer 6 having a p type ZnSe/ZnTe superlattice structure are successively epitaxially grown on a substrate 1 made of an n type GaAs or the like. A photoresist 10 is coated on the contact layer 6 and is then subjected to patterning by pattern exposure and development in a striped fashion in order to cover a region where a striped current path is to be formed. When using the photoresist 10 as a resist mask, the contact layer 6 is subjected to the patterning by chemical etching or the like so as to have a striped shape. At this time, until a part of each of the buffer layers 5 at both sides of the contact layer 6 is removed, the etching is performed.
Then, after the photoresist 10 is removed and then a polyimide 11 is coated on the whole surface, as shown in FIG. 1B, only the contact layer 6 is exposed by a proper photolithography technique.
Next, as shown in FIG. 1C, an electrode 8 formed of a multilayer film of Pd/Pt/Au or the like is formed by vacuum evaporation or the like so as to completely cover the polyimide 11 and the contact layer 6. Reference numeral 9 represents an electrode which is made of In or the like, and which is previously formed by a coating on a rear surface of the substrate 1, for example, or the like.
As described above, the polyimide 11 blocks the current, and the region sandwiched by the polyimide 11 is set as the striped current path, thereby constructing a semiconductor laser of a gain waveguide type.
The polyimide 11 has the advantages that it has excellent adhesion properties to ZnSe and to a p side electrode, it can be formed as a film by a process at a comparatively low temperature, and does not damage crystallinity of other portions.
However, the above-mentioned semiconductor laser of a gain waveguide type has the disadvantage that since most injected current is concentrated on a portion just beneath an electrode having a large contact resistance, it cannot be avoided that Joule heat generated around the electrode deteriorates the electrode portion and raises the temperature of the entire laser element.
There can be considered a countermeasure against this that the p side electrode is located at a lower portion of the element in order to suppress a rise of temperature of the element, and it is mounted on a heat sink having very high thermal conductivity. However, there is then the problem that since the polyimide 11 is provided on a portion occupying a comparatively large area between the p side electrode and the cladding layer, the heat cannot be effectively dissipated.